1. Field of the Invention
The present invention concerns an energy storage device, and, more particularly, an energy storage device comprising a pair of electrodes in contact with an electrolyte and a dielectric separator therebetween, wherein parasitic metal ions emanating from the electrode active material and disposed within the electrolyte are substantially removed from the electrolyte by adding a chelating material to the separator.
2. Description of Related Art
Energy storage devices typically comprise one or more pairs of electrodes separated by a dielectric layer, wherein one electrode (called a cathode) within a pair is adapted to store a positive charge, while the other electrode (called an anode) is adapted to store a negative charge. An electrolyte (typically in liquid form) allows an ionic current to flow between the electrodes and through the separator, while the same separator prevents an electric current (as opposed to an ionic current) current from shorting the energy storage device.
In certain types of energy storage devices, the electrodes are produced from carbon powder that is prepared from by-products of natural materials such as coconut shells, rice hulls, peat and coal for cost reasons. These natural materials inherently contain high levels of metal contaminants, particularly multivalent metal impurities, that leach into the electrolyte when an electro-chemical potential is applied between the electrodes during the operation of the energy storage device. Once in the electrolyte, these multivalent metal ions have the propensity to undergo redox electrochemical reactions between the electrodes whenever the cell potential is greater than the half cell redox reactions of the multivalent metal ions. The multivalent metal ion redox reactions therefore reduce the time dependent charge storage stability of the energy storage device, causing the phenomenon otherwise known as leakage current.
For instance, in a capacitor operating in a voltage range from −1.5V to +2V, the applied voltage is sufficient to cause the metal ions within the electrolyte to act as parasitic charge carriers between the anode and the cathode electrodes as they undergo half cell charge transfer reactions. As a consequence, these impurities drain away electrons from the electrodes through their redox couple reactions by undergoing reductions or oxidation reactions at the electrodes that reduce the charged voltage potential of the capacitor.
One solution to the parasitic ion problem is using higher purity carbon materials, such as polymeric resins or hydrocarbons, to manufacture electrodes. However, this solution is impractical for cost reasons.
Another solution is subjecting the carbon particles to extensive washing and thermal treatments aimed at reducing the total metal impurity content. However, these impurities are mostly contained within the bulk of the natural starting material, and these washing and thermal treatments are suited only to remove those impurities present on or near the surface of the carbon particles. Due to the electro-chemical potential on the carbon particles during capacitor operation, the metal contaminants contained in bulk or in the deep pores of the carbon material that are not accessible to the purification treatments migrate by diffusion to the surface of the particle and enter the electrolyte, contributing to the problematic self-discharge and leakage currents experienced in energy storage devices in the prior art.
Therefore, there is a need for an energy storage device wherein the movement of parasitic ions within the electrolyte is substantially restrained.